问题：
LaTex 中只有数学模式内置了希腊字母字体，比如μ，只能在$\mu$模式下使用斜体，此外xeLaTex编译时用siunitx包写的部分单位会报错
解决办法：
siunitx给出的解决办法如下
siunitx: Getting the micro symbol right
I get a few e-mails about siunitx and the micro symbol. People tend to be surprised that the symbol ‘sticks’ to a look very much like Computer Modern. The reason is that picking a proper upright (not italic) μ is not so easy in TeX. You don’t get one in Computer Modern, so siunitx takes one from the TS1 (text support) set in the absence of a better plan. I’ve set up some auto-detection for a few obvious alternatives (such as the upgreek package), but that doesn’t really work for XeTeX users.
XeTeX users are likely to load system fonts, and I’d hope be using UTF-8 input. That makes it hard to auto-detect what they are doing, but should make life easier for them to get things right. A lot of more comprehensive fonts include Greek letters in the main font, so getting the μ right is simple:
\sisetup{mathsmu =\text{μ}, textmu = μ}
or for people testing version 2 of siunitx:
\sisetup{maths-micro =\text{μ}, text-micro = μ}
此办法对我不起作用，直接改变Greek字母的字体更直接粗暴
CTAN下有些字体包，但其它地方发现一个fourier包的字体更好看，没有在这个目录下
直接
usepackage{fourier}
解决希腊字母问题，可以采样xeLaTex或者PDFLaTex编译，都不会出错
